The present invention relates to a method and apparatus for compensating chromatic dispersion and, in particular, a method and apparatus introducing positive and negative dispersion in a fashion to cancel out total dispersion in wavelength selective optical devices.
Systems which use optical components, exclusively or partially, for communicating information (typically digitally), switching, routing, transmitting and the like, generally provide certain advantages over, e.g. fully-electronic networks (e.g. providing typically higher data rates, requiring less physical space, less susceptibility to electromagnetic interference, and the like) but also present their own set of issues. These issues include signal loss and signal dispersion, each of which can occur either during transmission along optical fiber cables (or other transmission lines) or in discrete equipment or components such as optical routers, switches, hubs, bridges, multiplexers and the like. Certain types of components, such as erbium doped fiber amplifiers (EDFA) can provide sufficient amplification to overcome some or all transmission line losses, thus providing a system in which the limiting factor tends to be dispersion.
In general, dispersion refers to change or degradation of the wave shape of an optical signal, such as an (ideally) square-edged pulse. In general, the fact that different wavelengths have different effective rates of transmission along an optical transmission line and/or different indices of refraction and reflection can lead to pulse (or other signal) degradation, e.g. such that an original signal comprising a sequential plurality of square-edged pulses will, as a result of so called chromatic dispersion be changed such that each pulse, rather than retaining a substantially square-edged shape will have a more rounded, Gaussian shape. Dispersion can lead to, e.g. partial overlap between successive pulses resulting in signal detection problems such as high bit error rates, decrease in spectral efficiency or other problems, especially when combined with signal loss (amplitude reduction). Accordingly, it would be useful to provide a method and apparatus for use in optical systems, which can compensate for and/or reduce the amount of dispersion effect.
The dispersion problems become even more severe for wavelength division multiplexing (WDM) systems. The dense wavelength-division multiplexing (DWDM) scheme is widely adapted as one of the optimal solution to improve the bandwidth usage on optical fibers. By multiplexing multiple signals on different optical wavelengths, bandwidth of a single fiber can be multiple folded. Key optical components in DWDM systems include those which perform wavelength combining (multiplexing) and separating (demultiplexing) functions. The spectral response of the multiplexers and demultiplexers for DWDM applications are generally accompanied by certain dispersion effects that are determined by the underlying filtering technology. For example, the dispersion characteristic of a fiber Bragg grating can be determined by Hilbert transforming its transmission spectral response (e.g. as generally described in xe2x80x9cDispersion Properties of Optical Filters for WDM Systemsxe2x80x9d G. Lenz, B. J. Eggleton, C. R. Giles, C. K. Madsen, and R. E. Slusher, IEEE Journal of Quantum Electronics, Vol. 34, No 8 Page 1390-1402). The dispersion effects of wavelength multiplexing and filtering are very different from those of optical fibers. Optical fiber generally shows a linear dependency of its dispersion characteristic versus wavelength. Wavelength filters, multiplexers and demultiplexers, on the other hands, generally show nonlinear dispersion properties, e.g. correlated to its amplitude (spectral) response within its passband window. Although the accumulated dispersion due to fiber span can be compensated by different methods, such as dispersion compensating fibers or dispersion compensating fiber chirped gratings, dispersions caused by multiplexers/demultiplexers are difficult to compensate by conventional approaches. At least in narrow wavelength channel spacing DWDM systems that carry high data-rate information, it would be advantageous to provide dispersion filters, multiplexers, and demultiplexers that introduce minimum dispersion onto the signals.
The present invention includes a recognition of the existence, nature and/or source of certain problems in previous approaches, including as described herein. In one embodiment, the present invention involves the recognition that the chromatic dispersion occurring in a propagation path where polarization is intact or unchanged is substantially opposite to the dispersion along a similar propagation path but in which polarization is changed. According to one embodiment, the multi-stage or multi-component device is configured such that dispersion introduced at two different stacked waveplate filters along the optical path substantially cancel one another out, such as by introducing roughly equal amounts of positive and negative dispersion. In this context, dispersion values are approximately equal in magnitude if the difference in magnitude is sufficiently small that, upon combining oppositely-signed signals the resultant signal has a dispersion, in at least a first wavelength band of interest (such as a 90-95% transmission wavelength band) which is sufficiently low to achieve desired signal dispersion goals such as being less than about 10 ps, preferably less than 5 ps, more preferably less than about 3 ps and even more preferably less than about 2 ps. In one embodiment, chromatic dispersion which would otherwise be caused by stacked waveplate filters is (at least partially) canceled or compensated by the manner of arranging the optical signal propagation path.
A stacked waveplate device is described which provides dispersion with a first magnitude and a first sign for a first optical path having a first output polarization and which provides a second dispersion with a substantially equal but oppositely-signed dispersion for a second optical path defining an output having an orthogonal polarization to the polarization of said first output path. Optical paths are configured to pass through first and second stacked waveplate devices sequentially with the optical dispersion of said second device having an approximately equal magnitude but opposite sign compared to the optical dispersion of the first optical stacked waveplate device so as to provide canceling or compensation of optical dispersion. A device is configured to use cancellation or compensation of dispersion in sequential stacked waveplate devices to provide outputs with characteristics similar to outputs of previous stacked waveplate devices but with substantially reduced dispersion characteristics.